1,3-Propanediol (PDO) is used as a monomer unit for polyesters and polyurethanes and as a starting material for synthesizing cyclic compounds.
Various processes are known for the production of PDO via 3-hydroxypropanal (HPA) which start either from C2 and C1 structural units or from a C3 structural unit, such as, for example, acrolein. When acrolein is used, it is first hydrated in aqueous phase in the presence of an acidic catalyst to form HPA. After removing the unreacted acrolein, the aqueous reaction mixture formed during hydration still contains, in addition to 85 weight %, based on total organics, of HPA, approximately 8 weight % 4-oxaheptane-1,7-dial and further organic components in smaller proportions by weight. This reaction mixture is hydrogenated in the presence of hydrogenation catalysts to produce PDO. The PDO is recovered from the reaction mixture by distillation and/or extraction based methods known to those skilled in the art.
This invention resulted from the desire for an improved hydrogenation process for the conversion of 3-hydroxypropanal (HPA) to 1,3-propanediol (PDO) in the presence of a TiO2 supported ruthenium catalyst.
Hydrogenation processes may be characterized by the percent conversions of starting material to desired product, selectivities, and space-time yields achievable therewith. Percent conversion of HPA to PDO is defined by:
  X  =            %      ⁢                          ⁢      Conversion      ⁢                          ⁢      to      ⁢                          ⁢      PDO        =                            moles          ⁢                                          ⁢          of          ⁢                                          ⁢          PDO          ⁢                                          ⁢          formed                          moles          ⁢                                          ⁢          of          ⁢                                          ⁢          HPA          ⁢                                          ⁢          supplied                    ⨯      100      
Selectivity of the hydrogenation process is a measure of the amount of converted HPA which is converted into the desired product:
      %    ⁢                  ⁢    Selectivity    =                    moles        ⁢                                                  ⁢                                                ⁢        of        ⁢                                  ⁢        PDO        ⁢                                  ⁢        formed                    moles        ⁢                                  ⁢        of        ⁢                                  ⁢        HPA        ⁢                                  ⁢        converted              ⨯    100  
The space-time yield is another important characteristic for continuous hydrogenation processes, stating the achievable quantity of product per unit time and reaction volume.
When hydrogenating HPA to PDO on a large industrial scale, it is vital, with regard to the economic viability of the hydrogenation process and the quality of the product, for conversion to PDO and selectivity to be as close as possible to 100%. The PDO may be separated from the aqueous reaction media as well as from remaining HPA and secondary products contained in the product stream by distillation after the hydrogenation. However, this distillative separation is rendered very difficult by residual HPA and secondary products and may even become impossible due to reactions between the residual HPA and PDO to yield acetals, such as 2-(2′-hydroxyethyl)-1,3-dioxane (HED), which have a boiling point close to the boiling point of PDO. Thus, the lower the conversion and selectivity, the poorer the achievable product quality. Product streams containing excessive amounts of HED may be treated in a cleanup hydrogenation reactor to lower the HED level. Such post-treatment adds cost and investment. Thus, a catalyst that offers reduced HED levels offers significant economic advantage.
In order to produce PDO economically, it is also important for the catalyst to exhibit high activity for the hydrogenation of FPA. It is desirable to find a process which minimizes the quantity of catalyst required for the production of PDO.
Another important criterion for hydrogenation catalysts is their operational service life. Good catalysts lead to high conversion and high selectivity in the hydrogenation of HPA to PDO over an extended service life.
Conversion, selectivity, and space-time yield are influenced by the characteristics of the catalyst and by the hydrogenation conditions, such as reaction temperature, hydrogen pressure and duration of hydrogenation or, in the case of continuous hydrogenation, by the liquid hourly space velocity (LHSV).
U.S. Pat. No. 5,334,778 discloses a two-stage process for hydrogenating HPA which yields PDO having a residual carbonyl content, expressed as propanal, of below 500 ppm. The hydrogenation is carried out at 30° C. to 80° C. to a HPA conversion of 50 to 95% and then is continued at 100° C. to 180° C. to a HPA conversion of substantially 100%. Suitable hydrogenation catalysts therein include Raney nickel suspension catalysts, and supported catalysts based on platinum or ruthenium on activated carbon, Al2O3, SiO2, or TiO2 as well as nickel on oxide- or silicate-containing supports.
According to U.S. Pat. No. 5,015,789, very active nickel catalysts exhibit inadequate long-term stability, with a rapid drop in hydrogenation conversion and reaction speed upon repeated use of the catalyst. This results in frequent replacement of the entire catalyst packing, which is associated with known problems in the disposal and working up of compounds containing nickel. In addition, soluble nickel compounds can form and are released into the product stream, requiring further steps to separate the resulting contaminants.
U.S. Pat. No. 5,364,984 discloses a process for preparing PDO from HPA in an aqueous solution using a catalyst formed principally of titanium dioxide on which platinum is applied in a finely divided form. Preferred is a pyrogenic titanium dioxide obtained from titanium tetrachloride by flame hydrolysis, such as P25 (Degussa-Huils AG, Frankfurt am Main, Germany), presently available as Aeroxide® TiO2 P25 (Degussa Corporation, Piscataway, N.J.). The pyrogenic titanium dioxide is processed into shaped particles such as pellets, granulates, or extrusion molded particles and is then impregnated with the platinum, preferably using a soluble platinum compound such as hexachloroplatinic acid, and subsequently dried and reduced in a stream of hydrogen for 1 to 10 hours at temperatures of 250 to 500° C.
U.S. Pat. No. 6,232,511 B1 discloses a process for the production of PDO by the heterogeneously catalyzed hydrogenation of HPA. The catalyst is a supported catalyst which consists of an oxide phase, and on which is present ruthenium. The oxide phase may be TiO2, SiO2, Al2O3 and/or the mixed oxide thereof, such as aluminum silicate; MgO, zeolites and/or zirconium dioxide; or mixtures of oxide phases. Preferred oxide phases suitable as support materials include TiO2 and SiO2. The titanium dioxide (TiO2) used may be a pyrogenically produced titanium dioxide, such as Aeroxide® TiO2 P25 (Degussa Corporation, Piscataway, N.J.). The oxide phase may be coated by means of the incipient wetness method whereby the support is loaded with an aqueous ruthenium chloride solution, the loaded support is dried preferably at 20 to 100° C. in an inert gas atmosphere, and the dried impregnated support is then reduced with hydrogen to form metallic ruthenium, preferably at a temperature of 100 to 500° C. for a period of 20 minutes to 24 hours. The examples show preparation of a catalyst by reducing with a hydrogen at 200° C. for 8 hours a ruthenium impregnated titanium dioxide Aeroxide® TiO2 P25 (Degussa Corporation, Piscataway, N.J.) support.
There is still a need for a catalyst with: (a) excellent activity, (b) better catalyst selectivity—particularly lower 2-(2′-hydroxyethyl)-1,3-dioxane (HED) formation, (c) improved stability—longer lifetime in making PDO, and (c) improved ruthenium dispersion, enabling excellent activity, selectivity and stability with lower levels of ruthenium.